Radially expandable stents are commonly used to restore and maintain the patency of body passages, particularly blood vessels. Such stents are provided in a reduced diameter state suitable for percutaneous insertion into the body passage through a catheter. The stent is typically maintained in the reduced diameter state until delivery into the body passage, whereupon the stent is expanded to an increased diameter state within the body passage to hold open the body passage. A catheter delivered balloon is typically used to expand the stent to the increased diameter state.
A biocompatible cover can be provided on the outer and/or inner surfaces of the stent to reduce tissue reactions associated with the expansion of the stent into contact with the walls of the body vessel. Such covers can adversely effect the expansion characteristics of the stent by increasing the deployment pressure necessary to deploy the covered stent to the expanded state. Such covers can also cause asymmetrical expansion of the stent or incomplete expansion of portions or all of the stent. Additionally, deployment of the stent to the expanded state can cause the stent cover to tear and delaminate.
The present invention provides a covered stent that predictably and dependably expands to an increased diameter state at relatively low deployment pressures while concomitantly minimizing the risk of tearing of the stent covering during expansion. The stent covering is comprised of an inner cover and an outer cover that are positioned adjacent the inner surface and outer surface, respectively, of the stent structure to cover the stent. The inner cover and the outer cover can be constructed from the same or different biocompatible materials, such as, fluoropolymers like expanded polytetrafluoroethylene, having a structure of nodes interconnected by fibrils.
The inventors have determined that decreasing the radial thickness of the covering and increasing the average internodal distance (IND) of the fluoropolymers material forming the stent covering, reduces the deployment pressure necessary to expand the covered stent. However, they also increase the incidence of tearing of the stent covering.
In accordance with one aspect of the present invention, the stent covering has a radial thickness of at least about 0.008xe2x80x3 when the stent is in the reduced diameter, unexpanded state.
In accordance with another aspect of the present invention, the average internodal distance of the fluoropolymer material forming either the inner cover or the outer cover is greater than 100 microns when the stent is in the reduced diameter, unexpanded state. In one preferred embodiment, the average IND of the fluoropolymer material forming either the inner cover or the outer cover can be at least about 110 microns. In the most preferred embodiment, the average IND of the fluoropolymer material forming either the inner cover or the outer cover is at least about 135 microns.
In accordance with a further aspect of the present invention, the stent deploys from a reduced diameter configuration to an increased diameter configuration at an average deployment pressure of less than or equal to about 10 atm. In a preferred embodiment, the average deployment pressure of the stent is between about 4 atm and about 8 atm.
The present invention further provides a method for covering a stent structure. The method includes the step of positioning an inner cover of fluoropolymer material within the stent structure adjacent the inner surface of the stent structure and positioning an outer cover of fluoropolymer material over the outer surface of the stent structure. A compression tube is placed over the outer cover and the covered stent structure is heated to bond the inner cover to the outer cover.
The compression tube is preferably a tubular metal sleeve. In a preferred embodiment, a tube of fluoropolymer material is positioned between the outer cover and the compression tube. The compression tube and the tube of fluoropolymer material are removed after the covered stent structure is heated.
An adhesive can be applied to either the inner cover or the outer cover to facilitate bonding of the inner cover to the outer cover. The adhesive is preferably an aqueous dispersion of fluoropolymer material, such as PTFE. The aqueous dispersion can be heated after application to evaporate the aqueous component of the dispersion.